Magnetic recording/reproducing equipment for computers or the like generally drives a magnetic disk of the hard type having a magnetic layer on a rigid substrate relative to a flying magnetic head for magnetic recording/reproducing purposes. To meet the recent general demand for higher capacity and compactness, there is the desire for magnetic disk equipment to have a higher recording density. In order that magnetic disks have a higher recording density, attempts have been made for the development of thinner, smoother magnetic layers having a higher coercive force. For magnetic heads, in turn, great efforts have been made toward the narrowing of the gap, higher saturation magnetic flux density, and closer floating of the slider.
Most magnetic hard disks used in the past were of the coating type in which a magnetic coating composition containing magnetic powder and a binder is applied onto a substrate to form a magnetic layer. The coating type magnetic disks are generally manufactured by spin coating the magnetic coating composition to a substrate, orienting the magnetic powder, and curing the coating. The magnetic powder used in these magnetic disks is most often .gamma.-Fe.sub.2 O.sub.3 magnetic powder because of reliability. In turn, the flying magnetic heads used for the recording/reproduction of coating type magnetic disks include ferrite magnetic heads of the monolithic and composite types and thin film type magnetic heads. However, it is difficult to substantially increase the recording density of such disks even when combined with high performance magnetic heads because .gamma.-Fe.sub.2 O.sub.3 magnetic powder has a coercive force as low as 300 to 800 oersted (Oe).
To meet a demand for increasing the capacity of magnetic disks, magnetic disks of the thin film type now find increasing use. The thin film type magnetic disks have magnetic thin films, also known as continuous thin films, which are formed by such techniques as sputtering and plating and feature excellent electromagnetic properties and an increased recording density.
The thin film type magnetic disks most often use modified substrates including aluminum alloy substrates having an Ni-P undercoat layer plated thereon or a hard oxide layer formed by anodization. A Cr under layer, a metallic magnetic layer such as a Co-Ni layer, and a protective lubricant layer of carbon or the like are consecutively deposited on the substrate by sputtering.
Despite excellent electromagnetic properties and high density recording capability, the thin film type magnetic disks suffer from the problem that the magnetic layer is less durable and prone to failure upon repetitive contact-start-and-stop (CSS) cycles because it has increased surface energy and low hardness and is difficult to apply a lubricant layer thereon. Also, a reliability problem arises because adhesion or seizure often occurs between the disk and the head as a result of increased friction therebetween. These problems become significant as the flying height of the flying head, that is, the spacing between the magnetic disk surface and the opposing surface of the flying magnetic head is reduced.
Another problem of the thin film type magnetic disks is economy in that the magnetic layers are generally made of expensive materials and require costly equipment such as a vacuum vessel for their formation. Further, because of the multiple layered structure, the thin film type magnetic disks require a complex, time-consuming film forming process and are rather less adapted for efficient, mass scale production, leading to a further increase of cost.
Under these circumstances, one candidate for a durable or reliable magnetic disk amenable to efficient production in a mass scale and featuring a high recording density is a coating type magnetic disk using magnetic powder having a high coercive force, for example, ferromagnetic metal submicron particles and barium ferrite submicron particles.
In general, magnetic disks rely on the digital signal recording mode for carrying out saturation recording using 1 f and 2 f signals. In such recording and reproducing operation, a signal wave having a short wavelength of the order of 10 to 30 kFRPI is used as the 2 f signal for increasing the recording density. With the use of such short wavelength signal waves, if magnetic layers are relatively thick, there arise problems including difficulty of saturation recording and lowerings of overwrite ability and recording density. Therefore, the magnetic disks are required to have thin magnetic layers.
In the case of coating type magnetic disks, however, it is difficult to reduce the thickness of magnetic layers. The difficulty of formation of thinner magnetic layers is aggravated particularly when the magnetic powder is magnetic submicron particles having a high coercive force. Since such submicron particles have a very small particle diameter and enhanced magnetization, they tend to agglomerate and are less dispersible in magnetic coating compositions. In practice, with the spin coating technique, it is almost impossible to reduce the film thickness to below 0.5 .mu.m, especially below 0.3 .mu.m.
In addition, the following problems arise. (1) It is impossible to form a magnetic layer of uniform thickness by spin coating because the magnetic layer is relatively thinner on the disk center side and relatively thicker on the disk peripheral side due to centrifugal force. (2) The magnetic layer is relatively rough on the surface. (3) The magnetic layer is less even in quality and insufficient in orientation so that its squareness ratio is relatively low. (4) Because of the uneven coating thickness, errors occur more often as the average layer thickness is reduced.
One solution to these problems is the coating method proposed in Japanese Patent Publication No. 28644/1977 wherein a magnetic coating composition is spin coated in the air which has been passed through the same solvent as the solvent of the composition. This coating method prevents drying of the magnetic coating composition during spin coating, achieving some improvements in the unevenness of thickness due to the streaking upon spinning off. However, the method is insufficient in thickness reduction and surface roughness of the magnetic layer and achieves only a less improvement in the uneven thickness that the magnetic layer is relatively thinner on the inner diameter side and relatively thicker on the outer diameter side of the disk.
Particularly when magnetic coating compositions containing ferromagnetic metal submicron particles or barium ferrite submicron particles are coated, the agglomeration of particles makes it more difficult to form thinner layers. One potential approach is to first form a relatively thick magnetic layer and reduce the thickness by abrasion. Where it is desired to reduce the layer thickness to below 0.5 .mu.m, especially below 0.3 .mu.m, abrasion can cause thickness irregularities and flaws, leading to increased modulation and occurrence of more errors during recording and reproducing operation.
Japanese Patent Publication Nos. 36497/1981, 12209/1982 and 14888/1987 disclose the manufacture of magnetic disks. After a magnetic coating composition is applied to a substrate, orientation is effected on the coating by applying thereto a magnetic field for coplanar orientation and another magnetic field having a perpendicular component. With this method, magnetic particles vibrate during orientation, resulting in a more uniform coating with a higher squareness ratio. In addition, a thinner magnetic layer can be formed than in the absence of the perpendicular orientation magnetic field. However, problems also arise where it is desired to further reduce the layer thickness. Still thinner magnetic layers have relatively high surface roughness, insufficient orientation, and low squareness ratio, leading to increased modulation and error rate during recording and reproducing operation.
This tendency is aggravated particularly when magnetic coating compositions containing ferromagnetic metal. submicron particles and barium ferrite submicron particles are coated.
One approach to these problems is disclosed in Japanese Patent Application Kokai No. 248226/1986. A magnetic disk is manufactured by coating a substrate with iron oxide submicron particles, which have or have not been surface treated, to form a thin magnetic layer. Then, the iron oxide submicron particles in the layer are reduced to form a magnetic layer containing metallic submicron particles. The magnetic layer formed by this method, however, has an insufficient coercive force for high density recording. The magnetic layer is less resistant against weathering because it is difficult to form a stable passivated layer on the surface of metallic submicron particles.
Regarding coating type magnetic disks having magnetic layers with a thickness of up to 0.5 .mu.m, especially up to 0.3 .mu.m, the inventors have found that the electromagnetic properties and reliability of the magnetic disk system are largely affected by the surface roughness of both the magnetic layer and the substrate because the magnetic layer is quite thin. Since the magnetic disk system employs saturation recording, an increase in the surface roughness of the magnetic layer and substrate can directly affect the electromagnetic properties, giving rise to a serious problem, for example, a loss of S/N ratio during recording and reproducing operation. We have also found that the porosity of the magnetic layer largely affects the properties of the magnetic disk system.